Crdm with separate scram latch engagment and locking

ABSTRACT

A control rod drive mechanism (CRDM) configured to latch onto the lifting rod of a control rod assembly and including separate latch engagement and latch holding mechanisms. A CRDM configured to latch onto the lifting rod of a control rod assembly and including a four-bar linkage closing the latch, wherein the four-bar linkage biases the latch closed under force of gravity.

This application claims the benefit of U.S. Provisional Application No.61/792,235 filed Mar. 15, 2013 and titled “CRDM DESIGNS WITH SEPARATESCRAM LATCH ENGAGEMENT AND LOCKING”. U.S. Provisional Application No.61/792,235 filed Mar. 15, 2013 and titled “CRDM DESIGNS WITH SEPARATESCRAM LATCH ENGAGEMENT AND LOCKING” is hereby incorporated by referencein its entirety into the specification of this application.

This application was conceived in the course of work supported by theDepartment of Energy Cooperative Agreement No. DE-NE0000583. TheDepartment of Energy may have certain rights in this application.

BACKGROUND

DeSantis et al., U.S. Pub. No. 2011/0222640 A1 published Sep. 15, 2011and incorporated herein by reference in its entirety discloses (amongother subject matter) a CRDM for a nuclear reactor employing a leadscrew (sometimes referred to as a ball screw herein denoting specificlead screw embodiments employing ball nuts disposed between the screwand nut threadings) engaged by a motor to provide controlled verticaltranslation, in which a separate latch assembly connected with the leadscrew latches to the lifting rod of a control rod (or to the lifting rodof a control rod assembly comprising plural control rods connected by ayoke or spider to the lifting rod). The latch is actively closed toconnect the translating assembly comprising the lifting rod and thecontrol rod(s) so that the translating assembly translates with the leadscrew under control of the CRDM motor. Upon removal of the closingforce, e.g. during a SCRAM, the latch opens to release the lifting rodand SCRAM the control rod(s), while the lead screw remains engaged withthe CRDM motor and does not fall. In some illustrative embodiments, thelatches are actively closed by cam bars that are lifted by a hydraulicpiston, solenoid, or other lifting mechanism, where each cam bar is partof a four-bar linkage that moves the cam bar horizontally in response tothe lifting in order to cam the latches shut. In DeSantis et al., U.S.Pub. No. 2011/0222640 A1, the four-bar linkage is arranged such thatunder gravity the four-bar linkage operates to move the cam bars outwardso as to release the latch.

By way of non-limiting illustrative example, FIGS. 1 and 2 correspond todrawing sheets 1 and 16, respectively, of DeSantis et al., U.S. Pub. No.2011/0222640 A1. With reference to FIG. 1, an illustrative nuclearreactor vessel of the pressurized water reactor (PWR) type isdiagrammatically depicted. An illustrated primary vessel 10 contains areactor core 12, internal steam generator(s) 14, and internal controlrods 20. The illustrative reactor vessel includes four major components,namely: 1) a lower vessel 22, 2) upper internals 24, 3) an upper vessel26 and 4) an upper vessel head 28. A mid-flange 29 is disposed betweenthe lower and upper vessel sections 22, 26. Other vessel configurationsare also contemplated. Note that FIG. 1 is diagrammatic and does notinclude details such as pressure vessel penetrations for flow ofsecondary coolant into and out of the steam generators, electricalpenetrations for electrical components, and so forth. The lower vessel22 of the illustrative reactor vessel 10 of FIG. 1 contains the reactorcore 12, which can have substantially any suitable configuration. Theillustrative upper vessel 26 houses the steam generator 14 for thisillustrative PWR which has an internal steam generator design (sometimesreferred to as an integral PWR design). In FIG. 1, the steam generator14 is diagrammatically shown. In a typical circulation pattern theprimary coolant is heated by the reactor core 12 and rises through thecentral riser region 32 to exit the top of the shroud 30 whereupon theprimary coolant flows back down via the downcomer region 34 and acrossthe steam generators 14. Such primary coolant flow may be driven bynatural convection, by internal or external primary coolant pumps (notillustrated), or by a combination of pump-assisted natural convection.Although an integral PWR design is illustrated, it is also contemplatedfor the reactor vessel to have an external steam generator (notillustrated), in which case pressure vessel penetrations allow fortransfer of primary coolant to and from the external steam generator.The illustrative upper vessel head 28 is a separate component, but it isalso contemplated for the vessel head to be integral with the uppervessel 26. While FIG. 1 illustrates an integral PWR, in otherembodiments the PWR may not be an integral PWR, that is, in someembodiments the illustrated internal steam generators may be omitted infavor of one or more external steam generators. Still further, theillustrative PWR is an example, and in other embodiments a boiling waterreactor (BWR) or other reactor design may be employed, with eitherinternal or external steam generators.

With reference to FIG. 2, a control rod system embodiment is described,e.g. suitably part of the upper internals 24 of the nuclear reactor ofFIG. 1, which provides electromagnetic gray rod functionality (i.e.continuously adjustable control rod positioning) and a hydraulic latchsystem providing SCRAM functionality (i.e. in an emergency, the controlrods can be fully inserted in order to quickly quench the nuclearreaction, an operation known in the art as a SCRAM). The control rodsystem of FIG. 2 allows for failsafe SCRAM of the control rod clusterwithout scramming the lead screw. A motor/ball nut assembly is employed,such that a lead screw 40 is permanently engaged to a ball-nut assembly42 which provides for axial translation of the lead screw 40 by drivinga motor 44. The illustrative motor 44 is mounted on a standoff 45 thatpositions and bottom-supports the motor 44 in the support structure ofthe upper internals 24; other support arrangements are contemplated. Acontrol rod cluster (not shown) is connected to the lead screw 40 via alifting/connecting rod or lifting/connecting rod assembly 46 and a latchassembly 48. The lead screw 40 is substantially hollow, and thelifting/connecting rod 46 fits coaxially inside the inner diameter ofthe lead screw 40 and is free to translate vertically within the leadscrew 40. The latch assembly 48, with spring loaded latches, is attachedto (i.e. mounted on) the top of the lead screw 40. When the latches ofthe latch assembly 48 are engaged with the lifting rod 46 they couplethe lifting/connecting rod 46 to the lead screw 40 and when the latchesare disengaged they release the lifting/connecting rod 46 from the leadscrew 40. In the illustrated embodiment, latch engagements anddisengagements are achieved by using a four-bar linkage cam systemincluding two cam bars 50 and at least two cam bar links 52 per cam bar50. Additional cam bar links may be added to provide further support forthe cam bar. When the cam bars 50 move upward the cam bar links 52 ofthe four-bar linkage also cam the cam bars 50 inward so as to cause thelatches of the latch assembly 48 to rotate into engagement with thelifting/connecting rod 46. In the illustrated embodiment, a hydrauliclift assembly 56 is used to raise the cam bar assemblies 50. In analternative embodiment (not illustrated), an electric solenoid liftsystem is used to raise the cam bar assemblies. When a lift force isapplied to the cam system, the upward and inwardly-cammed motion of thecam bars 50 rotates the latches into engagement thereby coupling thelifting/connecting rod 46 to the lead screw 40. This causes the controlrod cluster to follow lead screw motion. When the lift force is removed,the cam bars 50 swing down and are cammed outward by the cam bar links52 of the four-bar linkage allowing the latches of the latch assembly 48to rotate out of engagement with the lifting/connecting rod 46. Thisde-couples the lifting/connecting rod 46 from the lead screw 40 whichcauses the control rod cluster to SCRAM. During a SCRAM, the lead screw40 remains at its current hold position. After the SCRAM event, the leadscrew 40 is driven to the bottom of its stroke via the electric motor44. When the lift force is reapplied to the cam system via the hydrauliclift assembly 56, the latches of the latch assembly 48 are re-engagedand the lifting rod 46 is re-coupled to the lead screw 40, and normaloperation can resume. Other latch drive modalities are contemplated,such as a pneumatic latch drive in which pneumatic pressure replaceshydraulic pressure in the illustrated lift assembly 56. In FIG. 2, thelead screw 40 is arbitrarily depicted in a partially withdrawn positionfor illustration purposes. The latching assembly 48 is attached to (i.e.mounted on) the top of the lead screw 40. The ball nut 42 and motor 44are at the bottom of the control rod drive mechanism (CDRM), the latchcam bars 50 extend for the full length of mechanism stroke, and thehydraulic lift system 56 is located at the top of the mechanism. In someembodiments, the CRDM of FIG. 2 is an integral CDRM in which the entiremechanism, including the electric motor 44 and ball nut 42, and thelatching assembly 48 are located within the reactor pressure vessel 10(see FIG. 1) at full operating temperature and pressure conditions.Further illustrative embodiments of CRDM designs employing the cam barswith four-bar linkages are described in DeSantis et al., U.S. Pub. No.2011/0222640 A1, which is incorporated herein by reference in itsentirety.

BRIEF SUMMARY

In some illustrative embodiments, a control rod drive mechanism (CRDM)comprises: a lead screw engaged by a CRDM motor; a lifting rodsupporting at least one control rod; latches secured to the lead screwand configured to latch an upper end of the lifting rod to the leadscrew; a latch engagement mechanism configured to close the latches ontothe upper end of the lifting rod; and a latch holding mechanismconfigured to hold the latches closed; wherein the latch holdingmechanism is separate from the latch engagement mechanism. In someembodiments the CRDM further comprises a four-bar linkage including cambars, the four-bar linkage configured to drive the cam bars inward tocam the latches closed responsive to operation of the latch engagementmechanism, the latch holding mechanism configured to hold the cam barsin the inward position to keep the latches closed. In some suchembodiments the four-bar linkage is configured to bias the latchesclosed under force of gravity. In some embodiments the latch engagementmechanism operates responsive to lowering the latches over the upper endof the lifting rod and is not effective to keep the latches closed whenthe latches are raised again after the latch engagement mechanismoperates.

In some illustrative embodiments, a control rod drive mechanism (CRDM)comprises: a lead screw engaged by a CRDM motor; a lifting rodsupporting at least one control rod; latches secured to the lead screwand configured to latch an upper end of the lifting rod to the leadscrew; a latch engagement mechanism configured to close the latches ontothe upper end of the lifting rod; and a latch holding mechanismconfigured to hold the latches closed; wherein the latch engagementmechanism is not effective to keep the latches closed when the latchesare supporting the weight of the lifting rod and supported at least onecontrol rod. In some embodiments the latch holding mechanism is noteffective to close the latches. In some embodiments the CRDM furthercomprises a four-bar linkage including cam bars, the four-bar linkageconfigured to drive the cam bars inward to cam the latches closedresponsive to operation of the latch engagement mechanism, the latchholding mechanism configured to hold the cam bars in the inward positionto keep the latches closed. In some such embodiments the four-barlinkage is configured to bias the latches closed under force of gravity.In some embodiments the latch engagement mechanism operates responsiveto lowering the latches over the upper end of the lifting rod and is noteffective to keep the latches closed when the latches are raised againafter the latch engagement mechanism operates.

In some illustrative embodiments, a control rod drive mechanism (CRDM)comprises: a lead screw engaged by a CRDM motor; a lifting rodsupporting at least one control rod; latches secured to the lead screwand configured to latch an upper end of the lifting rod to the leadscrew; and a four bar linkage including cam bars, the four bar linkageconfigured to drive the cam bars inward to cam the latches closedresponsive to operation of a latch engagement mechanism; wherein thefour bar linkage is configured to bias the latches closed under force ofgravity.

In some illustrative embodiments, a control rod drive mechanism (CRDM)includes: a CRDM motor; an element translated under control of the CRDMmotor;

a latch configured to latch a lifting rod supporting at least onecontrol rod with the element translated under control of the CRDM motor;a latch engagement mechanism configured to close the latch onto thelifting rod; and a latch holding mechanism, separate from the latchengagement mechanism, configured to hold the latch in its closedposition.

In some illustrative embodiments, a control rod drive mechanism (CRDM)includes: a CRDM motor; an element translated under control of the CRDMmotor; a latch configured to latch a lifting rod supporting at least onecontrol rod with the element translated under control of the CRDM motor;and a four bar linkage including cam bars, the four bar linkageconfigured to cam the latches closed responsive to operation of a latchengagement mechanism; wherein the four bar linkage is configured to biasthe latches closed under force of gravity.

In some illustrative embodiments, a control rod drive mechanism (CRDM)is configured to latch onto the lifting rod of a control rod assemblyand includes separate latch engagement and latch holding mechanisms.

In some illustrative embodiments, a control rod drive mechanism (CRDM)is configured to latch onto the lifting rod of a control rod assemblyand includes a four-bar linkage closing the latch, wherein the four-barlinkage biases the latch closed under force of gravity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and methods ofmanufacturing. The following is a brief description of the drawings,which are presented for the purposes of illustrating the exemplaryembodiments disclosed herein and not for the purposes of limiting thesame.

FIG. 1 diagrammatically shows a nuclear reactor illustrated in DeSantiset al., U.S. Pub. No. 2011/0222640 A1.

FIG. 2 diagrammatically shows a control rod system illustrated inDeSantis et al., U.S. Pub. No. 2011/0222640 A1.

FIG. 3 diagrammatically shows an isometric view of a CRDM with thecontrol rod fully inserted.

FIGS. 4 and 5 diagrammatically show isometric and side cutaway views,respectively, of the CRDM of FIG. 3 with the latching device disengaged.

FIG. 6 diagrammatically shows a side cutaway view of the CRDM of FIGS.3-5 with the latch engaged.

FIGS. 7-18 diagrammatically show aspects of a CRDM embodiment with aself-engaging cam/latch system and electromagnetic holding system asdescribed herein. With particular reference to FIG. 18 (inclusive ofFIGS. 18A-F): FIG. 18A illustrates a ball screw driving down, FIG. 18Billustrates latches at cam surface, FIG. 18C illustrates latches onconnecting rod OD, FIG. 18D illustrates ball screw continues down, FIG.18E illustrates latches reach pocket, and FIG. 18F illustrates latchesfull engaged.

FIGS. 19-22 diagrammatically show aspects of another illustrativeholding mechanism suitably used in, for example, the CRDM of FIGS. 3-6or the CRDM of FIGS. 7-18.

FIGS. 23-32 diagrammatically show aspects of another illustrativeholding mechanism suitably used in, for example, the CRDM of FIGS. 3-6or the CRDM of FIGS. 7-18.

FIGS. 33-38 diagrammatically show aspects of another illustrativeholding mechanism suitably used in, for example, the CRDM of FIGS. 3-6or the CRDM of FIGS. 7-18.

FIGS. 39-48 diagrammatically show aspects of another illustrative CRDMincluding a holding mechanism and a variant latching mechanism.

FIGS. 49-52 diagrammatically show aspects of another illustrativeholding mechanism suitably used in, for example, the CRDM of FIGS. 3-6or the CRDM of FIGS. 7-18.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Disclosed herein are improvements upon CRDM designs of DeSantis et al.,U.S. Pub. No. 2011/0222640 A1 employing the cam bars with four-barlinkages.

In one aspect, the CRDM is improved by separating the latch engagementand latch holding functions. This may entail increasing the number ofCRDM components since a separate latch engagement mechanism and latchholding mechanism are provided. However, it is recognized herein thatthis increase in parts is offset by improved energy efficiency. This isbecause the latch engagement is a momentary event that occurs veryinfrequently (possibly only once per fuel cycle). In contrast, the latchholding operation is performed over the entire fuel cycle (barring anySCRAM events). By employing separate latch engagement and holdingmechanisms, the latch holding mechanism is not required to perform therelatively higher-energy operation of moving the latches from theunlatched position to the latched position. Accordingly, the latchholding mechanism can be made more energy efficient.

In another aspect, the latch engagement mechanism, which no longer needsto perform the latch holding function, can be substantially improved. Inone embodiment (see FIGS. 3-6), the latch engagement mechanism comprisesa lower camming link built into the lower portion of the CRDM, which isengaged by the latch box or housing as it is lowered toward the liftingrod (which, due to its not currently being latched, is typically locatedat its lowermost position corresponding to maximum insertion of thecontrol rods into the nuclear reactor core). The lowering latch housingengages the lower camming link which is curved and mounted pivotally sothat an end distal from the end cammed by the latch housing is caused todrive the cam bars inward, into the latched position. Once in thelatched position, the separate latch holding mechanism is engaged, andthereafter when the latch housing is raised by the CRDM motor and leadscrew the lower camming link disengages but the latch remains closed byaction of the separate latch holding mechanism.

In another aspect, the latch engagement mechanism is implemented as aself-engaging cam/latch system (see FIGS. 7-18). This approach isachieved by modifying the four-bar linkage such that under gravity thefour-bar linkage operates to move the cam bars inward so as to engagethe latch. Similar to the latch engagement of FIGS. 3-6, this latchengagement activates upon lowering the latch housing over the upper endof the lifting rod. In the self-engaging approach, the latch is normallyclosed due to the four-bar linkage defaulting to moving the cam barsinward under force of gravity, and the upper end of the lifting rodincludes a camming surface that cams the latch open as the latch housingis lowered over the upper end of the lifting rod. Once over the cammingsurface of the upper end, the latch again closes under force of gravitydue to the orientation of the four-bar linkage. The separate latchholding mechanism is then activated to hold the cam bars in the inwardposition to keep the latch closed. Surprisingly, this embodiment iscapable of reliable SCRAM even though the four-bar linkage is biasingthe latch closed under gravity. This is because the four-bar linkage isdesigned with its links at large angles and of relatively long length sothat the force necessary to open the latches against the gravitationalclosing bias of the four-bar linkage is quite modest. (See FIGS. 7-18and related discussion for details). Accordingly, the weight of thetranslating assembly (i.e. the lifting rod and the attached control rodor rods and optional spider or yoke) is sufficient to easily overcomethe closing bias of the four-bar linkage.

In further disclosed aspects, various embodiments of the latch holdingmechanism are disclosed. See FIG. 19 and following.

In the CRDM system of FIG. 2, the lift system 56 (hydraulic as shown, oralternatively an electric solenoid) supports both latch actuation andlong term engagement during hold and translational operations. In thevariant embodiments described in the following, features of likefunctionality to the CRDM of FIG. 2 (for example, the cam bars 50 andthe cam bar links 52 of the four-bar linkage) are labeled with likereference numbers.

With reference to FIGS. 3-6 and with contextual reference to FIG. 2, aCRDM embodiment is described in which latch activation and long termhold/translation functions are separated, resulting in reduction ofoperational power requirements. The CRDM comprises a mechanicallyactuated latching device. FIG. 3 shows an isometric view of the CRDMwith the control rod (not shown) fully inserted. FIGS. 4 and 5 showisometric and side cutaway views, respectively, with the latching devicedisengaged. FIG. 6 shows a side cutaway view with the latch engaged. Thelatching mechanism utilizes the CRDM motor 44, the lead screw 40 (e.g.threadedly engaged with the CRDM motor 44 via the ball screw 42 as shownin FIG. 2) and a latch box 102 to engage the latches 104 to the top ofthe connecting (i.e. lifting) rod 46. Springs 106 bias the latches 104open. The latch box 102 and spring-biased latches 104 form a latchassembly corresponding to the latch assembly 48 of FIG. 2. In FIGS. 3-6,a mounting feature 108 is shown via which the latch box 102 is mountedto the top of the lead screw 40, but the lead screw itself is omitted inFIGS. 3-6. Similarly, only the top of the lifting rod 46 is shown inFIGS. 3-6, but it is to be understood that the lifting rod 46 extendsdownward as shown in contextual FIG. 2.) In this operation, the controlrod or rods are initially fully inserted and the upper end of thelifting rod 46 is disengaged from the latches 104.

The CRDM motor 44 is then operated to cause the lead screw 40 totranslate downward, thus lowering the latch box 102 toward the upper endof the lifting rod 46. The downward force supplied by the CRDM motor 44through the ball screw 42 moves the latch box 102 into contact with alower camming link 110 built into a lower portion 112 of the CRDM. FIGS.4 and 5 show isometric cutaway and side cutaway views, respectively, ofthe state in which the latch box 102 is just beginning to contact thelower camming link 110 at a contact area 114.

As seen in FIG. 6, the continued application of motor torque forces thelatch box 102 downward so as to press the lower camming link 110downward resulting in a rotary action about a pivot point 116. Thisrotary action lifts and translates the cam bars 50 into the engagedposition so as to cam against and close the latches 104 in the latch box102.

A separate holding mechanism (not shown in FIGS. 3-6 but embodiments ofwhich are disclosed elsewhere in this application) keeps the cam bars 50engaged as the latch box 102 is translated back upward after the latchengagement so as to lift the lifting rod 46 and attached control rod(s)upward. (Note that the control rods are not shown in FIGS. 3-6).

This approach of the embodiment of FIGS. 3-6 separates latch activationand long term hold/translation functions of the CRDM, resulting inreduction of operational power requirements. (Again, FIGS. 3-6illustrate only the latch activation—suitable embodiments of the longterm hold/translation component are described elsewhere in thisapplication.) The separation of latch activation and long termhold/translation functions simplifies the latching assembly making iteasier to manufacture and less expensive. The mechanically actuatedlatching device described with reference to FIGS. 3-6 is electricallyoperated (assuming the lead screw 40 is driven by the electric CRDMmotor 44 as per FIG. 2). In combination with an electrically operatedholding mechanism (again, disclosed elsewhere in this application), thisconstitutes an all-electric CRDM.

With reference to FIGS. 7-18, a CRDM embodiment with self-engagingcam/latch system and electromagnetic holding is described. In these CRDMembodiments, the four-bar linkage is modified such that under gravitythe four-bar linkage operates to move the cam bars 50 inward so as toengage the latch. These CRDM embodiments also include a holdingmechanism that only holds the latch and does not perform the engagement.

With reference to FIG. 7, the CRDM is shown in combination with acontrol rod assembly 140 connected by the lifting/connecting rod 46 viathe lead (or ball) screw 40 to the CRDM which includes the motorassembly 44, a modified cam assembly 144 (with a modified four-barlinkage) and latch assembly 148.

With reference to FIG. 8, an enlarged view of the CRDM of FIG. 7 isshown, including the motor 44 mounted on the standoff 45, the camassembly 144 with modified four-bar linkage, the latch assembly 148, andan optional position sensor 149. The illustrative CRDM also includes anelectromagnet holding system 150 at the top of the cam assembly 144.

With reference to FIGS. 9 and 10, which show cutaway perspective view ofthe CRDM in SCRAM mode (fully inserted) and in normal operating mode(translating or holding the control rods), respectively, the CRDM allowsfor failsafe SCRAM of the control rod (or control rod cluster) 140without the need to SCRAM the lead screw 40. The lead screw/ball nutassembly is permanently attached to the electric motor 44 (only the topof which is visible in FIG. 9) which provides for its axial translation.The control rod cluster 140 is connected to the lead screw 40 via aconnecting (i.e. lifting) rod 46 and the latch assembly 148 (see FIG.7). As seen in FIG. 9, the lead screw 40 is hollow, and the lifting rod46 fits inside the lead screw inner diameter (ID) and is free totranslate vertically within the lead screw 40. The latch assembly, withtwo latches 154 (although three or more latches are contemplated), issecured to the top of the lead screw 40 by a lead screw/latch assemblycoupling 156 (e.g., a latch housing mounted to the upper end of the leadscrew). When the latches 154 are engaged with the lifting rod 46 theycouple the lifting rod 46 to the lead screw 40 (normal operation) sothat the lead screw 40 and lifting rod 46 move together. When thelatches 154 are disengaged they release the lifting rod 46 from the leadscrew 40 (an event referred to as SCRAM).

Latch engagements and disengagements are achieved by using the four-barlinkage cam system 144 with a cam bar assembly provided for each latchincluding a cam bar 160 and cam bar links 162. However, unlike theembodiment of FIG. 2, in the CRDM embodiments of FIGS. 7-18 the cam barlinks 162 are oriented such that when gravity causes the cam bars 160 tomove downward the four-bar linkage action rotates the cam bars 160inward thereby causing the latches 154 to rotate into engagement withthe lifting rod 46. Because of this self-engaging feature, there is noaction required to engage the latches 154 to the lifting rod 46 (otherthan operating the CRDM motor 44 to lower the latch assembly 148 overthe upper end of the lifting rod 46) and there are no springs forbiasing the latches 154 (compare with springs 106 of the embodiment ofFIGS. 3-6).

Thus, force of gravity is sufficient to cause the cam bars 160 to camthe latches 154 to engage the lifting rod 46 when the lifting rod is inits lowermost position (corresponding to the control rods being fullyinserted). However, force of gravity is not capable of keeping thelatches 154 engaged when the CRDM of FIGS. 7-18 is operated to lift thecontrol rod assembly 140 via the lifting rod 46. Thus, the separateholding mechanism 150 is provided, which includes electromagnets 170 andmagnetic couplers 172 each connected with the upper end of a respectiveone of the cam bars 160. In the embodiments described herein withreference to FIGS. 7-18, the illustrative electromagnet holding system150 is incorporated to hold the cam bars 160, and thus the latches 154,in full engagement for long term hold and translational operations. Whenpower is removed from the electromagnets 170 (as per FIG. 9) the weightof the translating assembly 140, 46 is sufficient to rotate the latches154 and cams bars 160 out of engagement thereby causing the CRDM toSCRAM. (The term “translating assembly” or similar phraseology refers tothe combination of the lifting rod 46 and the control rod assembly 140including a set of control rods connected with the lifting rod 46 by ayoke or spider.) While the electromagnet holding mechanism embodiment150 is described for illustrative purposes in FIGS. 7-18, elsewhere inthis application other holding mechanism embodiments are disclosed thatmay be substituted for the holding mechanism 150.

After the SCRAM event the lead screw 40 is driven back to the bottom ofits stroke via the electric CRDM motor. As the latch assembly nears thebottom of the stroke it automatically re-engages with the lifting rod 46by cam action against the conical surface 176 of the upper end of theconnecting rod 46. The same automatic re-engagement action could also beused to re-engage in the event that a control rod becomes stuck and theSCRAM does not complete.

The overall CRDM assembly is shown in FIGS. 7-8. Note that the leadscrew 40 may also be referred to as a “ball screw”, which is anequivalent term when the threaded engagement employs a ball nut (thatis, a threaded nut/screw coupling with ball bearings disposed in thethreads). The layout of the CRDM of FIGS. 7-18 is similar toillustrative CRDMs described with reference to FIG. 2. However, in theCRDM of FIGS. 7-18 the electromagnet holding system 150 at the top ofthe CRDM has replaced the hydraulic (or solenoid) lift assembly 56 ofCRDM embodiments of FIG. 2.

FIG. 9 illustrates the CRDM of FIGS. 7-18 in full SCRAM mode with theball screw 40 and control rod assembly fully inserted. In FIG. 9 onlythe upper end of the lifting rod 46 (also sometimes called a connectingrod) is visible. The reversed (as compared with embodiments of FIG. 2)cam link orientation causes the four-bar linkage action under downwardgravitational weight of the cam bars 160 to rotate the cam bars 160inward into full engagement thereby causing the latches 154 to be fullyengaged with (the upper end of) the lifting rod 46 of the translatingassembly. This is the normal self-engaged cam bar position with no loadon the latches from the translating assembly and no electromagnetholding force applied by the electromagnet holding system 150.

FIG. 10 illustrates normal CRDM operation (either long term hold mode ortranslation of the control rod assembly under control of the CRDMmotor). For this operating condition the electromagnets 170 are poweredon to hold the cam bars 160, and thus the latches 154, in fullengagement so that they can carry the maximum translating assemblyweight force. As seen in FIG. 10, the cam bars 160 extend above the topplate of the cam housing where the magnetic couplers 172 are attached.These couplers 172, made of 410 SS magnetic material in a suitableembodiment, complete the magnetic circuit for optimum electromagnetholding force.

FIG. 11 shows the CRDM of FIGS. 7-18 at the start of SCRAM. The latches154 have been rotated out of engagement by the downward force due to theweight of the translating assembly. The latch heels, which are incontact with the cam bars 160, push the cam bars outward therebyallowing the connecting rod to SCRAM. This action is designated by theforce annotation 180 in FIG. 11. FIG. 11 shows the latches 154 in theland-on-land (LOL) position just riding over the outside diameter of theupper end of the connecting rod 46.

FIG. 12 illustrates the CRDM of FIGS. 7-18 with the latches 154 and cambars 160 in the fully disengaged position. This orientation is anon-operational position that could occur if the latches 154 are“kicked” outward by the downward movement of the translating assemblyduring SCRAM. Although this is a non-operational position with theself-engaged cam bar design of FIGS. 7-18, it illustrates that there isample clearance between the inside surface of the latches 154 and theconnecting rod 46 for SCRAM reliability. This is shown in the inset ofFIG. 12, where the clearance d_(clearance) is indicated.

FIG. 13 illustrates the force balance for SCRAM operation. In FIG. 13,the weight of the translating assembly is denoted W_(TA), the forcepushing the cam bars outward is denoted F_(push), and the weight of thecam bars is denoted W_(Cam Bar). In the illustrative design, the maximumforce needed to push each cam bar assembly outward for SCRAM (that is,the maximum required F_(push)) is only a few pounds. This lateral forcecomponent of the cam bar assembly weight W_(Cam Bar) can be minimized byincreasing the orientation angle of the cam link 162, e.g. to a minimumangle of about 70° in some calculated designs. In general, making thecam link 162 longer or at a larger angle (relative to the horizontal)reduces the maximum force needed to push out the cam bars. The minimumforce available to push each cam bar 160 outward is produced by latchrotation due to the downward weight force of the translating assembly.This minimum available force is based on the translating assembly weightW_(TA) minus worst-case assumed mechanical friction drag in the controlrod channel and worst-case friction at all contact surfaces. SCRAMreliability is assured since the minimum available force F_(push) forSCRAM is significantly larger than the force needed for SCRAM.Advantageously, the SCRAM is totally driven by gravity with no otherloads required.

FIG. 14 illustrates the force balance for normal operation. Sufficientlateral force F_(hold) must be applied at the heel of each latch 154 tohold the translating assembly weight W_(TA) for various modes ofoperation. In the illustrative embodiment of FIGS. 7-18, this force isprovided by the electromagnet holding system 150 at the top of the CRDM.Since the cam bars 160 are self-engaged, the cam bar side load reducesthe needed electromagnetic force. The minimum holding force F_(Mag)needed at the holding magnet 170 to maintain latch engagement duringtranslation of the control rod assembly is computed based on translatingassembly weight W_(TA) plus worst-case assumed mechanical friction dragin the control rod channel. In calculated designs, there is ampleholding force margin for all normal operating conditions.

FIG. 15 illustrates isometric views of the electromagnet holding system150 at the top of the CRDM. FIG. 15 shows the fully engaged operationalconfiguration (top view, power to magnet 170 either on or off), theSCRAM operational configuration (middle view, power to magnet 170 off)and the fully disengaged operational configuration (bottom view, powerto magnet 170 off). In the fully engaged mode (top view), either with orwithout electromagnet holding force, the magnetic couplers 172 areseated against the electromagnet housings 170. This seat provides theinward stop for the cam bars 160 and for the latches for fulloperational engagement.

FIG. 16 shows plan views corresponding to the isometric views of FIG.15. It is seen from FIG. 16 that for all operating modes theelectromagnet holding system 150 fits well within the CRDM spaceenvelope.

FIG. 17 illustrates an enlarged cutaway view of the electromagnetholding system 150 for the fully engaged condition. The electromagnets170 are suitably hermetically sealed by welding and potted for hightemperature use inside the reactor pressure vessel. Some suitablematerials for the components are as follows: for the electromagnet 170,the electromagnet housing may be alloy 625 non-magnetic material, theelectromagnet core may be 410 stainless steel magnetic material, and theelectromagnet winding may be 24 gauge copper wire; and the magnetcouplers 172 may suitably be 410 stainless steel magnetic material.Designs with these materials are estimated to provide a calculated 310lbs of holding force. These are merely illustrative examples, and othermaterials and/or design-basis holding force may be employed dependingupon the reactor design.

FIG. 18 illustrates the latch re-engagement action. The views arelabeled: (1) top left view; (2) top middle view; (3) top right view; (4)bottom left view; (5) bottom middle view; and (6) bottom right view.After a SCRAM event, when re-engagement is desired, the ball screw isdriven back to the bottom by the CRDM motor. The latches 154automatically re-engage with the lifting/connecting rod 46 as thelatching assembly reaches bottom. For this purpose, a conical camsurface 176 is incorporated into the configuration of the upper end ofthe connecting rod 46. As the latch assembly is driven back down, theinboard surfaces of the latches 154 slide down over the top of theconnecting rod 46, being cammed open by the conical cam surface 176against the gravitational bias toward closure driven by the four-barlinkage, until the self-engaged latches 154 snap back into the normalengagement pocket. Normal operation can then resume.

The same latch auto re-engagement action, as illustrated in FIG. 18, canalso be used to re-engage a control rod (or bank of control rods) thatbecomes stuck during SCRAM. The latch assembly is driven down over theupper end of the connecting rod 46 of the stuck rod (or rod bank) untilthe latches 154 snap into the normal engagement pocket. If it is desiredto fully insert the rods into the reactor core (as is typically the casein the event of a SCRAM), then the latching assembly is driven downwardby the ball screw and motor with the latches 154 pushing downward on thestuck rod. In that scenario, the bottom surfaces of the latches 154contact the flat portion of the engaging pocket in the connecting rod46. As load is applied, the eccentricity of the contact surfaces causesthe latches 154 to remain engaged without any additional holding system.As the motor drives the ball screw down, the latches drive the stuck rodin.

With reference to FIGS. 19-22, another holding mechanism embodiment fora CRDM is described. In this regard, FIGS. 3-6 and 7-18 illustrateembodiments in which latch activation and long term hold/translationfunctions are separated, resulting in reduction of operational powerrequirements. FIGS. 3-6 illustrate an embodiment of the latchactivation, while FIGS. 7-18 illustrate an embodiment of the latchactivation (the self-engaging cam/latch system) in combination with anembodiment 150 of the long term hold/translation function. FIGS. 19-22illustrate another embodiment of the long term hold/translationfunction, which may be used in combination with the embodiment of FIGS.3-6 or substituted for the holding mechanism 150 of the embodiment ofFIGS. 7-18.

FIG. 19 shows an isometric view of the latch hold mechanism of FIGS.19-22 operating in conjunction with the cam assembly of FIGS. 2-6, i.e.with cam bars 50. FIGS. 20 and 21 show side view and cutaway side views,respectively, of the latch hold mechanism in its disengaged position.FIG. 22 shows a side cutaway view of the latch hold mechanism in itsengaged position. The holding mechanism illustrated in FIGS. 19-22utilizes a large electromagnet 200, coupled with a magnetic hanger 202connected with the upper ends of the cam bars 50 by pins 204, as shownin FIG. 19. The electromagnet 200 is spaced apart from the hanger 202 bysupport posts 206 extending from a base plate 208 secured to (orforming) the top of the cam bar assembly 144. With the CRDM engaged byan engagement mechanism (such as that described with reference to FIGS.3-6, in illustrative FIGS. 19-22), the electromagnet 200 is activated,causing a magnetic attraction between the hanger 202 and theelectromagnet 200 that holds the hanger 202 in contact with theelectromagnet 200 as shown in FIG. 22 (or, in alternative embodiments,into contact with a landing surface interposed between the electromagnetand the hanger). The raised hanger bar 202 holds the cam bars 50 intheir raised (i.e. engaged) position via the pins 204. When power is cutto the electromagnet 200 the attractive force between the magnet 200 andthe hanger 202 is severed, causing the hanger 200 and cam bars 50 tofall to the disengaged position shown in FIGS. 20 and 21. Pin slots 210in the hanger 202 accommodate the lateral motion of the cam bars 50 dueto the four-bar linkage. The sectional views of FIGS. 21 and 22illustrate the copper windings 212 of the electromagnet 200.

By separating latch activation and long term hold/translation functionsof the latch of the CRDM, it is recognized herein that the operationalpower requirements can be reduced, since the holding mechanism is notrequired to actually lift the cam bars, but merely maintains the cambars in the lifted position after the (different) engagement mechanismoperates. The separation of features simplifies the holding featuremaking it easier to manufacture and less expensive.

With reference to FIGS. 23-32, another holding mechanism embodiment fora CRDM is described, which may be used in combination with theembodiment of FIGS. 3-6 or substituted for the holding mechanism 150 ofthe embodiment of FIGS. 7-18. FIGS. 23-25 show two isometric views and aplan view, respectively, of the holding mechanism in the fully engagedposition. FIGS. 26-28 show two isometric views and a plan view,respectively, of the holding mechanism in the SCRAM position. FIGS.29-31 show two isometric views and a plan view, respectively, of theholding mechanism in the fully disengaged position. The isometric viewof FIGS. 23, 26, and 29 show the top region of the CRDM including theholding mechanism at a viewing angle of approximately 45°. The isometricview of FIGS. 24, 27, and 30 show the top region of the CRDM includingthe holding mechanism at a more oblique viewing angle than 45°.

FIG. 32 illustrates a plan view of the holding mechanism withannotations of the electromagnet holding force F_(Elect) for applying aforce F_(Cam Bar) sufficient to hold the cam bars 160.

The holding mechanism of FIGS. 19-28 utilizes horizontal holding arms230 that have slots 232 into which pins 234 at the tops of the cam bars160 (e.g. cam bar pins 234) fit. When the cam bars 160 are moved to theengaged position by an engagement mechanism (e.g. such as the onedescribed with reference to FIGS. 3-6, or the self-engaging cam/latchsystem of the embodiment of FIGS. 7-18), the cam bar pin 234 in each pinslot 232 pushes the holding arm 230 to rotate to a point where it is inclose proximity with an electromagnet 240. The rotation is about an armpivot point 242, and the various components of the holding mechanism aremounted on a baseplate 244 that is secured to (or forms) the top of thecam bar assembly 144. When power is applied to the electromagnets 240they attract and hold the arms 230 which are made of magnetic material.The restrained arms, in turn, hold the cam bars 160 in the engagedposition via the cam bar pins 234 in the pin slots 232 and therebymaintain latch engagement. FIGS. 23-25 shows two alternative isometricviews and a top view, respectively, of the holding mechanism in thisfully engaged position.

With reference to FIGS. 26-28 (SCRAM mode) and FIGS. 29-31 (fullydisengaged mode), when power is cut to the electromagnets 240, theattractive force between the electromagnets 240 and the arms 230 issevered, allowing the arms 230 to rotate out of engagement. The weightof the translating assembly is sufficient to disengage the latches andmove the cam bars 160 away (i.e. outward) for SCRAM. During this action,the holding arms 230 freely move out of the way.

With particular reference to FIG. 32, the holding mechanism of FIGS.23-32 provides a mechanical advantage due to the configuration of theholding arms 230. This is accomplished by the relative positions of thearm pivot point 242, the cam bar contact point (i.e. the engagementbetween the cam bar pin 234 and the pin slot 232) and the electromagnetholding force contact point (corresponding to the location of theelectromagnet 240), suitably quantified by the distance d_(mag) betweenthe magnet 240 and the pivot point 242 and the distance d_(pin) betweenthe cam bar contact point (approximately the cam bar pin 234) and thepivot point 242. Because of this mechanical advantage, the holding forceF_(Elect) provided by the electromagnets 240 can be reduced to provide agiven force F_(Cam Bar) for holding the cam bars 160. This facilitatesthe use of smaller, less complex electromagnets as the electromagnets240, as well as lower power demands for operation.

The configuration of the electromagnetic holding mechanism of FIGS.23-32 will vary somewhat depending on the configuration of the cam bars160 and the four bar linkage. The pin slot 232 is arranged toaccommodate the horizontal cam bar travel while providing theappropriate engagement to rotate the horizontal holding arms 230.

In a variant embodiment, magnets are embedded into the holding arms toprovide added holding strength. In some embodiments, this added force isexpected to be enough to enable the holding mechanism of FIGS. 23-32 toperform both the engagement and holding operations, and could, forexample, be used in place of the hydraulic lifting assembly 56 of theembodiment of FIG. 2.

By way of review, FIGS. 23-25 show the cam bars 160 and holding arms 230in the fully engaged position, either held by the electromagnets 240 orengaged by an outside means (e.g. such as the one described withreference to FIGS. 3-6, or the self-engaging cam/latch system of theembodiment of FIGS. 7-18) prior to powering the electromagnets 240.FIGS. 26-28 show the SCRAM mode, in which the arms 230 and thus the cambars 160 have moved sufficiently for the latches to completely releasethe connecting (i.e. lifting) rod and control rod assembly. FIGS. 29-31show the fully disengaged position. Due to the 4-bar linkage action, thecam bars 160 rise and fall as they are moved laterally from engaged todisengaged positions. This action is best seen in the isometric view ofFIGS. 24, 27, and 30. Since the holding arms 230 pivot about fixedsupport posts (the pivot arm points 242), the pin slots 232 areincorporated into the holding arms 230 to accommodate the rise and fallof the cam bars 160. These slots 232 should be sized and positioned toaccommodate both the rise and fall of the cam bars 160 and the lateralmotion of the cam bars 160 due the four-bar linkage action responding tothe rise/fall of the cam bars 160.

When used in conjunction with the self-engaging cam/latch systemdescribed herein with reference to FIGS. 7-18, the direct mechanicaladvantage for the illustrated locations of the holding arm pivot points242 has been estimated to be approximately 4.5:1 (corresponding to theratio d_(mag)/d_(pin) in FIG. 32). However, there is not a directrelationship between this mechanical advantage and the holding forceneeded since the holding arms 230 do not pull in line with the plane ofcollapse of the cam bars 160. A force correction is needed that isproportional to the cosine of the holding arm angle. The net effect forthe configuration shown herein is an effective mechanical advantage of2.4:1. This force balance, along with the effective mechanicaladvantage, is diagrammatically illustrated in FIG. 32. The holdingmechanism of FIGS. 23-32 has the benefit of a mechanical advantageprovided by the configuration of the holding arms.

With reference to FIGS. 33-38, another holding mechanism embodiment fora CRDM is described, which may be used in combination with theembodiment of FIGS. 3-6 or substituted for the holding mechanism 150 ofthe embodiment of FIGS. 7-18. FIGS. 33-35 show two isometric views atdifferent viewing angles and a top view, respectively, of the top of theCRDM (and more particularly the top of the cam assembly and the holdingmechanism) with the cam system in the unlatched position. FIGS. 36-38show two isometric views at different viewing angles and a top view,respectively, of the top of the CRDM including the holding mechanismwith the cam system in the latched position. Illustrative FIGS. 33-38show the holding mechanism in combination with the embodiment of FIGS.3-6, and hence the cam bars are labeled cam bars 50 in FIGS. 33-38.

Once the cam system is in the engaged (i.e. “latched”) position theholding mechanism of FIGS. 33-38 holds the cam bars 50 such that theyengage the latches and maintain latching of the connecting (i.e.lifting) rod. The holding mechanism of FIGS. 33-38 includes two hightemperature magnets 260 and magnetic links 262 attached to the upper endof each of the two cam bars 50 at the top end of the CRDM. The twocanned high temperature electromagnets are suitably wired in a parallelfashion.

When the cam system transitions from the unlatched position (FIGS.33-35) to the engaged (latched) position (FIGS. 36-38), the upper endsof the cam bars 50 engaging the magnetic links 262 rotate the magneticlinks 262 about pivots 264 so that the ends 270 of the magnetic links262 distal from the cam bar/magnetic link joint 272 are moved by theinward movement of the cam bars 50 to be in close proximity to theelectromagnets 260. When the electromagnets 260 are energized thesedistal ends 270 of the magnetic links 262 are held against the magnets270, and the cam bar 50 at the opposite end of the link 262 is preventedfrom moving. This holds the latch in the latched position. The holdingpower of the electromagnets 260 is adequate to hold the weight of thecam bars 50 as well as the force exerted on the cam bars 50 by thelatches. The latched state is shown in alternative isometric views(FIGS. 36 and 37) and a plan view (FIG. 38). Slots 276 in a base plate278 secured to (or forming) the top of the cam bar assembly andsupporting the hold mechanism components accommodate the lateral motionof the cam bars 50 during unlatched/latched transitions.

When used in conjunction with the embodiment of FIGS. 3-6 (asillustrated in FIGS. 33-38), operation is as follows. When theelectromagnets 260 are de-energized the magnetic links 262 are decoupledfrom the electromagnets 260 and the cam bars 50 are free to fall undertheir own weight and swing into the unlatched position. In the unlatchedposition the cam bars 50 are disengaged from the latches and the latchescan then rotate out of engagement with the connecting rod. When the cambars 50 are disengaged from the latches, the latches can be rotated outof engagement with the connecting rod by the latch springs 106 (for theembodiment of FIGS. 3-6). Therefore, in the unlatched position the cambars 50 are not engaged with the latches, the latches are not engagedwith the lifting rod and the translating assembly (including the liftingrod and the attached control rod or rods) can then fall under its ownweight (SCRAM). The holding mechanism of FIGS. 33-38 is fail-safe in thesense that if power is lost to the electromagnets 260 the connecting rodwill SCRAM due to gravity.

Operation of the holding mechanism of FIGS. 33-38 in conjunction withthe cam arrangement of FIGS. 7-18 (self-latching) is similar, exceptthat when the electromagnets 260 are de-energized the cam bars 160 donot open under gravity, but rather are cammed open by the cam surface atthe upper end of the lifting rod 46 of the falling translating assembly.(See description of FIGS. 7-18 for details). Again, the de-energizing ofthe electromagnets 260 allows the magnetic links 262, and hence the cambars 160, to freely move to perform the SCRAM.

With reference to FIGS. 39-48, another holding mechanism embodiment fora CRDM is described, which may be used in combination with theembodiment of FIGS. 3-6 or substituted for the holding mechanism 150 ofthe embodiment of FIGS. 7-18. The embodiment of FIGS. 39-48 isillustrated in conjunction with a four-bar linkage with cam bars and cambar links oriented as in the embodiments of FIGS. 2-6; accordingly, inFIGS. 39-48 the cam bars and cam bar links are labeled as cam bars 50and cam bar links 52, respectively.

The embodiment of FIGS. 39-48 illustrates a variant latching mechanismlocated beneath the cam assembly, in which a hydraulic cylinder 300 (or,alternatively, an electric solenoid) raises a lift plunger or piston 302upward to engage cam bar lift rollers 304 at the bottom ends of the cambars 50 so as to raise the cam bars 50—by action of the four-bar linkageprovided by cam bar links 52 this raising of the cam bars 50simultaneously moves the cam bars 50 inward to engage the latch. (Bycomparison, in the embodiment described with reference to FIG. 2, thehydraulic lift assembly 56 located above the cam assembly lifts theupper ends of the cam bars 50 to engage the latches). The embodiment ofFIGS. 39-48 also illustrates a holding mechanism located above the camassembly, where a base plate 308 secured to (or forming) the top of thecam bar assembly supports the hold mechanism components.

FIG. 39 shows a diagrammatic side view of the cam assembly, in which thelift system (comprising electric solenoid or hydraulic cylinder 300 andpiston 302 in conjunction with cam bar lift rollers 304) is deactivatedand the hold mechanism (diagrammatically shown in a tilted view) is alsodeactivated. FIG. 40 shows a top view of the deactivated hold mechanismcorresponding to FIG. 39. FIG. 41 shows a diagrammatic side view of thecam assembly in which the lift system is activated and the holdmechanism is still deactivated. FIG. 42 shows a top view of thedeactivated hold mechanism corresponding to FIG. 41. FIG. 43 shows adiagrammatic side view of the cam assembly in which both the lift systemand the hold mechanism are activated, and FIG. 44 shows a correspondingtop view of the activated hold mechanism. FIG. 45 shows a diagrammaticside view of the cam assembly in which the lift system is deactivatedand the hold mechanism is still activated, and FIG. 46 shows acorresponding top view of the activated hold mechanism. FIGS. 47 and 48illustrate geometric aspects of the hold mechanism.

The hold mechanism of the embodiment of FIGS. 39-48 keeps the four-barlinkage cam system 50, 52 in the engaged position during rod translationand hold functions, and provides the SCRAM functionality whensubsequently deactivated. It also structurally internalizes the majorityof the cam bar retention force required to hold the latches in theengaged position, and utilizes mechanical advantage to minimize theremaining hold force, resulting in a structurally efficient unit.

FIGS. 39 and 40 illustrate the holding mechanism (and associated liftsystem in FIG. 39) both in the deactivated state. The holding mechanismincluding a rotary hold bar 310, a hold-solenoid 312 (where the housingof the solenoid 312 is visible), a hold-solenoid plunger 314, andhold-bar rollers 316, is located at the top or base plate 308 of the cambar assembly. FIGS. 39 and 40 illustrate the hold mechanism deactivatedat startup. Prior to startup, the lift system (electric solenoid orhydraulic), which includes the electric solenoid or hydraulic cylinder300 and the lift plunger or piston 302, is also deactivated. Therefore,the latches are not engaged by the four-bar cam system 50, 52, renderingthe connecting rod and attached control rods in the fully insertedposition. As best seen in the top view of FIG. 40, in the unlatchedstate of the four-bar linkage 50, 52 the cam bars 50 are in theiroutboard positions (i.e., moved outward and away from the latches). Alsonote that the base plate 308 includes slots to accommodate movement ofthe upper ends of the cam bars 50 between their inboard (i.e. moved in)and outboard (i.e. moved out) horizontal positions.

With reference to FIGS. 41 and 42, upon activation of the lift system(shown in FIG. 41), the lift plunger or piston 302 raises the cam bars50 into the latch engagement position by contact with the cam bar liftrollers 304. At initial engagement of the lift mechanism, the holdmechanism is still deactivated as depicted in FIGS. 41 and 42. Becauseof activation of the lift system, the latches are now engaged with theconnecting rod which is resting with the attached control rods at thefully inserted position. As best seen in FIG. 42, the lifting of the cambars 50 also moves the cam bars 50 into their inboard positions byaction of the four-bar linkage, and this inward movement is what engagesthe latches, as described in more detail with reference to theembodiments of FIGS. 2-6.

With reference to FIGS. 43 and 44, subsequently following activation ofthe lift system, the hold solenoid 312 of the hold mechanism isactivated, resulting in extension of the solenoid plunger 314, whichrotates the hold bar 310 about a pivoting engagement 318 of the hold bar310 with the base plate 308. At full extension of the solenoid plunger314, the hold-bar rollers 316 are rotated into position behind the upperextremity (i.e. upper ends) of the cam bars 50 (note again that theupper ends of the cam bars 50 protrude through the slots in the baseplate 308), so as to function in the hold capacity. It is noted that thehold solenoid 312 is free to pivot about a post mount 320 that securesthe solenoid 312 on the base plate 308. It is also noted that thesolenoid plunger 314 is pin-connected to the hold bar 310, whichprovides rotational freedom for operation. The relative orientations ofall the pertinent components at this phase of operation are illustratedin FIGS. 43 and 44.

With reference to FIGS. 45 and 46, with the hold mechanism activated thelift system can be deactivated, with the hold system thereafter keepingthe latches engaged. Upon deactivation of the lift system, the liftplunger or piston 302 is released, and therefore, no longer (bottom)supports the cam bars 50. At this point, the cam bars 50 are retained inthe engaged position solely by the hold mechanism. The four-bar camsystem 50, 52 is now being retained for long-term retention of theconnecting rod by the hold mechanism.

With reference to FIG. 47, there exists an eccentricity E_(contact)between the center of rotation of the hold bar 310 and the line ofaction of the contact force between (the upper end of) the cam bar 50and the hold-bar roller 316. This eccentricity E_(contact) results in aforce-moment imbalance on the hold bar 310 when the force applied by thehold solenoid 312 is removed. This moment imbalance at power loss to thehold solenoid 312 is the driving mechanism for rapidly rotating the holdbar 310 and the attached rollers 316 out of contact with the cam bars50—resulting in SCRAM (rapid release of connecting rod). In order tocreate a smooth rolling action of the hold-bar rollers 316 on thecontact surface of the cam bars 50, the contact surface is contoured tothe arc of the rolling-contact point.

With continuing reference to FIG. 47 and with further reference to FIG.48, the desired lower power consumption of the hold mechanism is aproduct of the significant mechanical advantage of the unit. The momentarm E_(plunger) of the hold solenoid plunger 314, relative to the pivotcenter of the hold bar 310, is significantly larger than the moment armof the contact force of the cam bar 50 at the hold-bar roller 316, asillustrated in FIGS. 47 and 48. Therefore, the force required by thehold solenoid 312 is significantly less than the latch-to-cam barcontact force required to support the connecting rod load. Of furtheradvantage, internalization of the majority of the cam bar retentionforces as equal and opposite loads reacted through the hold bar 310eliminates force reaction through the remainder of the hold mechanism,resulting in a structurally efficient unit.

As previously stated, the hold mechanism described with reference toFIGS. 39-48 separates latch activation and long term hold/translationfunctions, resulting in reduction of operational power requirements. Thehold mechanism keeps the four-bar linkage cam system in the engagedposition during rod translation and hold functions, and provides theSCRAM functionality when subsequently deactivated. It also structurallyinternalizes the majority of the cam bar retention force required tohold the latches in the engaged position, and utilizes mechanicaladvantage to minimize the remaining hold force, resulting in astructurally efficient unit.

With reference to FIGS. 49-52, another holding mechanism embodiment fora CRDM is described, which may be used in combination with theembodiment of FIGS. 3-6 or substituted for the holding mechanism 150 ofthe embodiment of FIGS. 7-18. FIG. 49 shows an isometric view of the topregion of the CRDM including the holding mechanism with the verticallinkage engaged to raise the cam bars. FIG. 50 shows a correspondingisometric view with the vertical linkage disengaged to allow the cambars to fall. FIG. 51 corresponds to the engaged view of FIG. 49 butincludes a partial cutaway, and similarly FIG. 52 corresponds to thedisengaged view of FIG. 50 but includes the partial cutaway.

The latch holding mechanism of FIGS. 49-52 utilizes a vertical linkagesystem including vertical links 340 connected to a hanger 342 disposedbetween (the upper ends of) the cam bars 160 of FIGS. 7-18 (as shown;or, alternatively, the cam bars 50 of FIGS. 2-6) and (in the engagedposition shown in FIGS. 49 and 51) held in the engaged position byelectromagnets 344. When the cam bars 160 are moved to the engagedposition by the separate latch engagement mechanism (e.g. as in theembodiment of FIGS. 3-6, or the embodiment of FIGS. 7-18), it causes thehanger 342 to move up which, in turn, raises the vertical links 340 to aposition where horizontal drive members 348 are in close proximity withthe electromagnets 344. When power is applied to the electromagnets 344they attract and hold magnets that are embedded into the horizontaldrive members 348. (Alternatively, the horizontal members 348 may bemade of steel or another ferromagnetic material but not includemagnets). The restrained vertical links 340, in turn, hold the hanger342, and thus the cam bars 160, in the engaged position and therebymaintain latch engagement.

When power is cut to the electromagnets 344, the attractive forcebetween the electromagnets 344 and the horizontal drive members 348 issevered, allowing the vertical links 340 to drop out of engagement, asseen in FIGS. 50 and 52. The weight of the translating assembly issufficient to disengage the latches and move the cam bars 160 away forSCRAM. During this action, the linkage system freely moves downward outof the way.

To recap, FIGS. 49 and 50 show isometric views of the top region of theCRDM at a viewing angle of approximately 45° for the engaged anddisengaged states, respectively. FIG. 49 shows the vertical linkagesystem in the fully engaged (full up) position, either held by theelectromagnets 344 or engaged by an outside means prior to powering theelectromagnets. For the SCRAM mode, shown in FIG. 50, the linkage systemhas moved full down for the latches to completely release the connectingrod and control rod assembly.

FIGS. 51 and 52 show isometric cutaway views of the top region of theCRDM for the engaged and disengaged states, respectively. FIG. 51 showsthe vertical linkage system in the fully engaged (full up) position,either held by the electromagnets 344 or engaged by an outside meansprior to powering the electromagnets 344. FIG. 52 shows the linkagesystem in the full down (SCRAM) position.

In the illustrative embodiment, the minimum angle of the vertical links340, in the fully engaged position (FIGS. 49 and 51), is set to about10° which is expected to assure an adequate SCRAM reliability margin. Inthe disengaged position (FIGS. 50 and 52) the vertical links 340collapse to a maximum angle of about 40° in the illustrative embodiment.

The latch holding mechanism described with reference to FIGS. 49-52provides a mechanical advantage due to the configuration of the linkagesystem. This is due to the relative positions and size of the verticallink 340 lengths compared to the horizontal drive member 348. Inaddition, the permanent magnet that is embedded in the horizontal arm348 provides added holding force. The true mechanical advantage for thisdisclosed vertical linkage system is calculated to be 2.9:1 at theminimum link angle. However, the effective mechanical advantage ishigher, estimated to be closer to 4.0:1, when an assumed permanentmagnet force per link assembly is added. Because of this mechanicaladvantage, the required holding force needed by the electromagnets isreduced. This results in smaller, less complex electromagnets, as wellas lower power demands for operation.

Illustrative embodiments including the preferred embodiments have beendescribed. While specific embodiments have been shown and described indetail to illustrate the application and principles of the invention andmethods, it will be understood that it is not intended that the presentinvention be limited thereto and that the invention may be embodiedotherwise without departing from such principles. In some embodiments ofthe invention, certain features of the invention may sometimes be usedto advantage without a corresponding use of the other features.Accordingly, all such changes and embodiments properly fall within thescope of the following claims. Obviously, modifications and alterationswill occur to others upon reading and understanding the precedingdetailed description. It is intended that the present disclosure beconstrued as including all such modifications and alterations insofar asthey come within the scope of the appended claims or the equivalentsthereof

1. A control rod drive mechanism (CRDM) comprising: a lead screw; alifting rod; latches secured to the lead screw and configured to latchan upper end of the lifting rod to the lead screw; a latch engagementmechanism configured to close the latches onto the upper end of thelifting rod; and a latch holding mechanism configured to hold thelatches closed; wherein the latch holding mechanism is separate from thelatch engagement mechanism.
 2. The CRDM of claim 1 further comprising: afour-bar linkage including cam bars, the four-bar linkage configured todrive the cam bars inward to cam the latches closed responsive tooperation of the latch engagement mechanism, the latch holding mechanismconfigured to hold the cam bars in the inward position to keep thelatches closed.
 3. The CRDM of claim 2 wherein the four-bar linkage isconfigured to bias the latches closed under force of gravity.
 4. TheCRDM of claim 1 wherein the latch engagement mechanism operatesresponsive to lowering the latches over the upper end of the lifting rodand is not effective to keep the latches closed when the latches areraised again after the latch engagement mechanism operates.
 5. The CRDMof claim 4 wherein the latch holding mechanism is located at a top ofthe CRDM and is configured to engage the upper ends of the cam bars tohold the cam bars in the inward position.
 6. The CRDM of claim 5 whereinthe latch holding mechanism comprises a magnetic coupling including anelectromagnet that when energized magnetically holds the cam bars in theinward position.
 7. The CRDM of claim 5 wherein the latch holdingmechanism comprises elements configured to move in a horizontal planeresponsive to a holding force applied by an actuator to hold the cambars in the inward position.
 8. A nuclear reactor comprising: apressurized water reactor (PWR) including: a pressure vessel; a reactorcore disposed in the pressure vessel; and a CRDM as set forth in claim 1disposed in the pressure vessel.
 9. The nuclear reactor of claim 8wherein the latch holding mechanism is magnetically operated.
 10. Thenuclear reactor of claim 8 comprising multiple CRDMs, wherein each CRDMhas an independent latch holding mechanism.
 11. The nuclear reactor ofclaim 10 wherein the latch holding mechanisms are magnetically operated.12. The nuclear reactor of claim 11 wherein the latch holding mechanismoperate independent of one another.
 13. A control rod drive mechanism(CRDM) including: a CRDM motor; an element translated under control ofthe CRDM motor; a latch configured to latch a lifting rod supporting atleast one control rod with the element translated under control of theCRDM motor; a latch engagement mechanism configured to close the latchonto the lifting rod; and a latch holding mechanism, separate from thelatch engagement mechanism, configured to hold the latch in its closedposition.
 14. The CRDM of claim 13 wherein the latch holding system isoperatively connected to a four-bar linkage and the four-bar linkage isconfigured to cam the latches closed responsive to operation of thelatch engagement mechanism.
 15. The CRDM of claim 14 wherein the latchholding system is magnetically actuated.
 16. The CRDM of claim 14wherein the latch holding system is operatively connected to a four-barlinkage and the four-bar linkage is configured to bias the latchesclosed under force of gravity.
 17. The CRDM of claim 16 wherein thelatch holding system is magnetically actuated.